Regarding to energy resources, China features being abundant in coal and deficient in oil and gas, and coal has accounted for nearly 70% of the total primary energy consumption for a long period, whereas the gap of oil and natural gas supply has become larger year by year. At the same time, the rapid development of China economy has created huge demand for organic chemicals, especially petroleum based chemicals, such as benzene, p-xylene, phenol, ethylene, etc. Hence, China should pay full attention to the use of its abundant coal resource, which can be transformed into scarce gas/oil fuels and chemicals to make up for the shortage of gas and oil resources.
Pyrolysis provides a mild conversion process to produce alternative oil and gas from solid fuels, such as coal, biomass, and so on. The so-called coal pyrolysis refers to the complex process wherein coal undergoes a series of physical changes and chemical reactions occurred at different temperatures after being heated under the condition of air isolation, resulting in pyrolysis gas (also known as gas), pyrolysis oil (also known as coal tar or tar), solid (char or coke) and other products. The pyrolysis of coal is also called as carbonization or thermal decomposition of coal. The coal pyrolysis process is a key step in the transformation of coal. During any one of the processes of gasification, liquefaction, coking and combustion of coal, pyrolysis reactions occur. Though the gas/oil production rate of pyrolysis of coal is far lower than those of gasification and liquefaction of coal, the coal pyrolysis process possesses many advantages from the point of view of production of alternative oil/gas and chemicals, such as mild conversion condition, short process flow, wide adaptability of coal type, high energy efficiency and low water consumption.
Since the 1970s, outside of China, many technologies for coal pyrolysis have been developed. The typical pyrolysis processes adopted rotary kiln, moving bed, fluidized bed and entrained flow reactors. The rotary kiln was used in Toscoal and Encoal processes developed by USA, and also in ATP process developed by Canada. The Toscoal and Encoal processes mainly aimed at upgrading of low rank coals, wherein target product was solid fuel char and the gas or fuel oil produced in the process were used as heating fuels. The ATP process focused on extracting oil from oil shale and the core equipment therein was a horizontal rotary kiln with multiple partitions, so the size of the equipment was large. A moving bed was used in LFC process developed by USA, Lurgi-Ruhr process developed by Germany, ETCH-175 process developed by the former Soviet Union. The LFC process implemented three-step treatment of coal, namely drying, pyrolysis and solid product finishing, wherein the heat source was the gas produced in the process and the property of char was stable after passivation treatment. The Lurgi-Ruhr process used char as a heat carrier in a moving bed pyrolyzer with mechanical agitation, and its advantages were high oil yield and low energy consumption, but the produced tar contained a large amount of dust and its discharging system was prone to be blocked, and also the wear problem became very serious due to the mechanical agitation device. The ETCH-175 process used hot powder char as a heat carrier, which led to a high tar yield but the tar contained high heavy fractions. The COED process adopted multistage fluidized bed by connecting a plurality of fluidized beds in series to realize sectional rapid heating. In the COED process, most of the tar is produced in low temperature region and the secondary pyrolysis reactions are greatly reduced. Furthermore, about 40-50% of the gas product was H2 which can be used in the hydrogenation of the tar. The COED process was complex and the produced tar contained a lot of fine powder char which should be removed by filter. The CSIRO process used solid and gas heat carriers to supply heat and produced char and liquid fuels from lignite. The entrained flow reactor was used in Garrett pyrolysis process developed by USA. In the Garrett process, the pulverized coal with particle size less than 0.1 mm was mixed with high temperature heat carrier of char in the entrained flow reactor to realize reactions of coal pyrolysis by fast heating. The char was combusted with air to supply heat for the pyrolyzer. The advantages of Garrett process was its high tar yield, while its disadvantage was that the generated tar and char dust were prone to adhere to the cyclone and the inner wall of the pipe, leading to pipe blockage after long time running.
Since the 1990s, inside of China, many technologies for coal pyrolysis process and poly-generation process based on coal pyrolysis have been developed. Beijing Research Institute of Coal Chemistry of China Coal Research Institute has developed multistage rotary furnace (MRF) pyrolysis process. The MRF process mainly comprised three horizontal rotary kilns connected in series, i.e., internally heated rotary drying furnace, externally heated rotary pyrolysis furnace and quenched coke furnace, respectively. Since most of the moisture in coal was removed before pyrolysis, the process greatly reduced the amount of waste water containing phenol. Moreover, the waste water containing phenol was mixed with fresh water to be used for coke quenching, so as to avoid establishing the high cost waste water treatment system. Dalian University of Technology (DG) developed a solid heat carrier pyrolysis process which consisted of coal preparation, coal drying, coal pyrolysis, powder char heating riser, combustion of mixture of coal and char in fluidized bed, gas cooling, transporting and purification. The core technology of DG process used char as solid heat carrier and performed material treatment and heat transfer based on the amount of heat needed for coal pyrolysis in the way of fluidization. The Institute of Process Engineering, Institute of Engineering Thermophysics, Institute of Coal Chemistry of Chinese Academy of Sciences and Zhejiang University have respectively developed poly-generation processes integrated circulating fluidized bed combustion and coal pyrolysis reactor, which adopted downer, moving bed and fluidized bed pyrolyzers, respectively. The processes used the hot ash of circulating fluidized bed boiler as heat carrier to provide heat for coal pyrolysis to extract oil and gas fuels, obtaining high value chemicals contained in the coal before coal combustion, which can realize cascade utilization of coal.
The Institute of Chemical Metallurgy of Chinese Academy of Sciences invented a process and an apparatus for four kinds of products generation by solid circulating fluidization using solid hydrocarbon fuel (Patent No. ZL01110152.0), wherein fuel oil and fuel gas were firstly extracted from solid powder fuel by fast pyrolysis in circulating fluidized bed reactor, and then the produced char was burned to produce heat/electricity. This process can only apply to powder materials with particle size of 0.03-0.3 mm. The Zhejiang University disclosed a poly-generation apparatus and a method for co-producing heat, electricity, gas and tar in a circulating fluidized bed (Patent No. ZL200610154581.X), wherein oil and gas were extracted from pyrolysis of coal heated by hot ash from the circulating fluidized bed combustor and residue char was burned in the circulating fluidized bed combustor through combination of circulating fluidized bed combustion and fluidized bed pyrolysis. The Institute of Engineering Thermophysics of Chinese Academy of Sciences disclosed a fast pyrolysis method and an apparatus using solid heat carrier (Publication No. CN101353582A) in a circulating fluidized bed, which adopted a hot ash divider to realize stable distribution of high temperature circulating ash between coal combustion and pyrolysis loop. The Institute of Engineering Thermophysics of Chinese Academy of Sciences also disclosed a coal topping method (Publication No. CN101435574A), which provided a pyrolysis and gasification equipment using solid heat carrier in front of the circulating fluidized bed boiler, feeding some or all of the coal into the equipment to carry out coal topping to produce pyrolysis gas and tar, the char produced was fed to the circulating fluidized bed boiler or combustion chamber to be burned. The above-mentioned technologies belong to solid heat carrier pyrolysis process which should be coupled with circulating fluidized bed boiler, thereby subject to the insufficient retrofit space of old boilers. The Shenhua Group Corporation Limited disclosed a coal pyrolysis method for upgrading of coal (Publication No. CN101608125A). Raw coal with the size less than 30 mm was fed into a rotary dryer to realize drying of raw coal through direct heat exchange by contact with the hot flue gas and meanwhile indirect heat exchange with hot char from a rotary pyrolyzer. The dried raw coal mixed with high temperature char from the heating rotary kiln was fed into the pyrolyzer, wherein the coal was directly heated by the high temperature char to reach final pyrolysis temperature of 500-700° C. This invention used the char heat carrier to supply heat in the rotary furnace, which resulted in difficulty for hot char circulation and transport. The Institute of Process Engineering of Chinese Academy of Sciences disclosed a method and an apparatus (Publication No. CN101781583) for high value utilization of coal by pyrolysis and gasification. The coal was partially or totally pyrolyzed before gasification to separately realize pyrolysis and gasification procedures through combination of dilute-phase transport bed and dense-phase fluidized bed, so as to realize co-generation of pyrolysis gas, gasification gas and pyrolysis oil. The invention adopted the gas heat carrier to realize the coupling of pyrolysis and gasification, but the amount of fine char powders entrained in the gas was high due to the adoption of transport bed in the upper stage, so the separation of gas and solid products was a difficult problem to overcome.
At present, all of the pyrolysis processes developed globally are in pilot or industrial demonstration stage, there is yet no large-scale commercial pyrolysis plant to produce alternative oil and gas from coal. The core technical problem is the control of yield and quality of oil and gas in the pyrolysis process. Many pilot and industrial test results show that the pitch content in the tar produced in most of the pyrolysis process is greater than 50% at present. The high fraction of heavy components in the tar not only reduces the quality and the value of coal-derived oils but also causes ineffective separation of high viscosity tar and dust entrained in the system, which causes a series of operation problems and hinders the industrial application of pyrolysis technology. Therefore, it is urgent to control the quality of pyrolysis product in order to produce a large amount of alternative fuel oil, natural gas and chemicals in the industrialization of coal pyrolysis process.